1. Field of the Invention
The present invention relates to a catalyst for cleaning up exhaust gas discharged from engines and more specifically to a monolithic catalyst for cleaning up exhaust gas, whose cleanup capacity is highly improved.
2. Description of the Prior Art
The requirements for the improvement of heat resistance and cleanup capacity of catalysts for exhaust gas cleanup have increasingly been strict recently. Recently, there have likewise been required for the development and use of exhaust gas-cleanup catalysts for motorcycles. For this reason, there have been required for the improvement of these catalysts in not only the exhaust gas-cleanup quality in the three way point, but also cleanup quality for hydrocarbons within the rich-region.
Up to now, as a catalyst excellent in heat resistance, there has been proposed an exhaust gas-cleanup catalyst prepared by wash-coating the surface of a carrier with a slurry comprising an element of the platinum group, active alumina, cerium oxide, a barium compound and a zirconium compound, followed by drying and calcination (see, for instance, Japanese Unexamined Patent Publication (hereinafter referred to as "J. P. KOKAI") No. Sho 64-58347. In addition, Japanese Patent Application Serial (hereinafter referred to as "J.P.A.") No. Hei 3-207446 discloses a catalyst in which heat deterioration of CeO.sub.2 is prevented and the heat resistance thereof is improved through fixing at least one member selected from lanthanum and barium on CeO.sub.2 capable of showing oxygen-storing effect (i.e., it absorbs oxygen when the O.sub.2 concentration increases or the air-fuel ratio is in the lean side, while it releases oxygen when the O.sub.2 concentration decreases or the air-fuel ratio is in the rich side and thus it contributes to the catalytic reaction).
However, the foregoing catalysts for exhaust gas cleanup are improved only in the oxygen-storing effect and heat resistance around the theoretical mixing ratio and insufficient in the ability of eliminating hydrocarbons and CO gas within the rich-region. Therefore, there has been required for the development of a catalyst having a good ability of eliminating hydrocarbons and CO within the rich-region as well as high durability.